1) Field of the Invention
The present invention relates to an optical scanning apparatus, an image forming apparatus, and a beam positioning method.
2) Description of the Related Art
One of the methods to increase recording speed of an image forming apparatus such as a laser printer or a digital copier is to increase rotation speed of a polygon mirror of an optical scanning apparatus that is used in a laser recording system of the image forming apparatus. However, with this method, improvement of the recording speed is limited due to problems with durability, noise, and vibration of a motor, as well as modulating speed of a laser. To cope with the problem, an optical scanning apparatus with a function of recording a plurality of lines simultaneously by scanning a plurality of beams at a time has been proposed. A light source that can emit a plurality of light beams at a time is called as a multi-beam light source.
A light source with a semiconductor laser array is one of the examples of the multi-beam light source, including a plurality of light-emitting points, i.e. light-emitting channels in one package. However, it is difficult to increase the number of channels in a manufacturing process of the semiconductor laser. It is also difficult to eliminate the effect of thermal and electrical crosstalk, and to shorten the wavelength. For these reasons, the multi-beam light source of this type is supposed to be expensive.
On the other hand, in a single-beam semiconductor laser the shortening of the wavelength is comparatively easier and it can be manufactured at a low cost. For these reasons, the single-beam laser has been used generally in various industrial fields. A large number of multiple-beam scanners and light sources in which the single-beam semiconductor laser or the multi-beam semi conductor laser is used as a light source and a plurality of laser beams is combined by using a beam-combining unit, have been proposed. A four-drum tandem image forming apparatus which is described below is an example.
In recent years, with a demand for a high-speed color image forming apparatus, a digital copier or a laser printer in which four photosensitive drums are arranged in a direction of transfer of a recording paper to achieve a color image, has been put to practical use. The digital copier or a laser printer with four photosensitive drums includes a plurality of scanning optical systems corresponding to each of the photosensitive drums. The scanning optical systems expose the photosensitive drums simultaneously to form latent images. A developing unit that uses toners of different colors such as yellow, magenta, black, and cyan, visualizes the latent images. The visualized images are transferred by superimposing one after another on a transfer paper and thus a color image is formed. This is called as four-drum tandem image forming apparatus.
The four-drum tandem image forming apparatus is favorable for outputting a printed image at high speed since as compared to a single drum image forming apparatus, it is capable of forming and outputting both a color and a monochrome image with the same speed. However, on the other hand, since it is necessary to provide four scanning optical systems corresponding to four photosensitive drums, reducing the size of the apparatus remains to be a problem. Moreover, a technical problem of reducing the color shift when the toner images that are developed at the photosensitive drum as the image carriers are transferred to the recording medium by superimposing remains to be solved. The color shift is in a main scanning direction and a second scanning direction.
Following are the causes of the color shift in the secondary scanning direction:                (1) Unevenness in feeding speed in a circumferential direction (secondary scanning direction) of the photosensitive drum;        (2) Unevenness in feeding speed in a circumferential direction (secondary scanning direction) of an intermediate transferring body;        (3) Positional error between the photosensitive drums;        (4) Shift in beam-spot writing position between the scanning optical systems; and        (5) Shift in position due to environmental changes like (1) to (4) or temperature changes during continuous printing.        
While performing writing by multi-beam on all photosensitive drums simultaneously, since rotations of a polygon scanner which is a light deflector and feeding speed of the photosensitive drum which are not synchronized in general, there is a possibility of a shift in the number of beams only in the secondary scanning direction.
The following method has been known for reducing the color shift.
Firstly, a technology in which, in an image forming apparatus that uses a plurality of scanners, scanning lines in all the photosensitive drums are matched by adjusting all the scanning units or the overall housing corresponding to the photosensitive drums is disclosed in Japanese Patent Application Laid-open Publication No. 2001-133718. However, according to the conventional technology disclosed, the mechanism required for the adjustment is complicated and it takes time for the adjustment. Further, since the housing which is heavy in weight is adjusted, it is difficult to deal with changes with the lapse of time such as a change in temperature, and precise correction of the color shift during printing or in the environment where the apparatus is used, cannot be carried out.
Secondly, as another solution to the problems mentioned above, a method of controlling the position of the secondary beam by using a galvanic mirror is disclosed in Japanese Patent Application Laid-open Publication No. 2001-100127. However, since the sensitivity of the galvanic mirror is too high to control the position of the secondary scanning beam, it is affected easily by the external vibrations. For this reason, to secure an appropriate beam spot, a profile irregularity as high as about four times that of the transmitting surface is sought.
Thirdly, as an invention to solve the problem of shift between the multi-beams, a color image forming apparatus characterized by including a correcting unit to correct the color shift, is disclosed in Japanese Patent Application Laid-open Publication No. H10-239939. In this color image forming apparatus, the correcting unit carries out correction of the color shift by adjusting the starting position of image writing (writing-start position) for each color in a secondary scanning direction by changing to a laser beam that writes an image on the photosensitive drum first, from among the plurality of laser beams in accordance with a phase relationship of a intermediate transfer reference signal and a line synchronization signal. However, even by using this type, the minute correction of less than one line cannot be carried out. Therefore, for writing 600 dots per inch (dpi), for example, there is a color shift of at least 42 μm.
Conventionally, there has been a method of increasing the rotational speed of the polygon mirror which is a deflecting scanning unit that improves the recording speed in an optical sensor that writes an image by emitting a light beam on a medium to be scanned, has been used. However, due to problems such as durability, noise and vibrations of a motor, and modulating speed of laser, there are limitations on the rotational speed that can be set. To overcome this problem, there is a method of recording a plurality of lines simultaneously by scanning a plurality of light beams at a time.
The multi-beam light source that emits a plurality of laser beams, in which a multi-beam semiconductor laser (for example, a semiconductor laser array) which includes a plurality of light emitting points (light emitting channels) in one package, is one of the examples of the multi-beam light source. However, it is difficult to increase the number of channels in the manufacturing process of the semiconductor laser. It is also difficult to eliminate the effect of thermal and electrical crosstalk, and to shorten the wavelength. For these reasons, the multi-beam light source of this type is supposed to be expensive.
On the other hand, in a single-beam semiconductor laser, the shortening of the wavelength is comparatively easier and it can be manufactured at a low cost. For these reasons, the single-beam semiconductor laser has been used generally in various industrial fields. A large number of multiple-beam scanners and light sources in which the single beam semiconductor laser (or the multi-beam semiconductor laser) is used as a light source and a plurality of laser beams is combined by using a beam-combining unit, have been proposed.
Thus, a method of combining a plurality of laser beams by using the beam-combining unit has many advantages regarding aspects such as shortening of wavelength and reducing the cost. Another advantage is that the adjustment (setting) of a distance between beam spots (beam pitch; distance between scanning lines) in a secondary scanning direction in a surface subjected to scanning, can be carried out easily by deflecting minutely the direction of emission of each laser beam.
While using the semiconductor laser array as a light source, it is necessary to rotate the semiconductor laser array roughly around optical axis to change the distance between the beam spots. Therefore, the amount of light becomes insufficient due to distribution of a divergence angle of a laser beam that is emitted from the semiconductor laser array and it may be difficult to reduce the diameter of the beam spot in the surface subjected to scanning to a smaller diameter.
However, as compared to a method of using the semiconductor laser array as a light source, in a method of combining the plurality of laser beams by using the beam-combining unit, the direction of emission tends to change easily due to the effect of the environmental changes and changes in elapsed time, and the distance between the beam spots in the surface subjected to scanning tend to change.
A conventional technology to solve these problems, in which the light beams emitted from the plurality of light sources are combined by using a beam-combining prism, is disclosed in Japanese Patent Application Laid-open Publication No. H10-215351. According to this patent document, the position of beam spot on the surface subjected to scanning is adjusted by adjusting the direction of emission of the light beam by shifting a cylindrical lens which forms a line image on a reflecting surface of the polygon mirror.
A technology in which the light beams emitted from the plurality of light sources is combined by using a half mirror in a multi-beam scanning and the multi-beam scanner is disclosed in Japanese Patent Application Laid-open Publication No. H9-189873. According to this technology, the position of the beam spot on the surface subjected to scanning is adjusted by adjusting the direction of emergence of the light beam by adjusting an inclination of the light source and an inclination of a galvanic mirror that is disposed in an optical path.
A technology to deflect a laser beam by constructing an optical deflector by using a change in the refractive index of an opto-electric material (such as lithium niobate) that has an opto-electric effect is disclosed in Japanese Patent Application Laid-open Publication No. H 10-282531.
A method of adjustment of light beam, a multi-beam scanner, and an optical scanning apparatus are disclosed in Japanese Patent Application Laid-open Publication No. 2002-174785. According to the method of adjustment of light beam, the multi-beam scanner, and the optical scanning apparatus, two dioptric systems without an image forming function are disposed in an optical path of at least one light beam from among a plurality of light beams from a light source that has a plurality of light emitters. At least one of position and direction of a light beam that is transmitted through the dioptric system is adjusted by changing spatial condition of the dioptric system.
A progress is being made in using optical elements made from plastic in an optical writing system to reduce the cost. So far, all optical elements made from glass have been used and the optical elements were coated to prevent reflection. However, if the optical elements made from plastic are used, there are various problems from technical as well as cost point of view in coating the optical elements made from plastic and there has been a tendency to omit coating process.
For this reason, an amount of ghost light due to reflection between surfaces of lenses which doesn't cause any problem in a case of a glass prism which is coated to prevent reflection, is large in a case of the plastic lens for which the coat to prevent reflection is omitted. Further, due to an improved sensitivity of photosensitive drums in recent years, the ghost light appears as an image, thereby deteriorating the image quality. Particularly, in a case of the ghost light that occurs irrespective of the rotation of the deflector, since the same position is irradiated without being scanned, although it is a minute amount of energy, the amount of light is substantial when accumulated. This light appears as an image, thereby deteriorating the image quality.
As a measure to deal with the ghost light, various technologies have been proposed so far. A method to prevent the ghost light before the deflector is disclosed in Japanese Patent Application Laid-open Publication No. H9-274134. According to this method, the ghost light is prevented before the deflector by using an arrangement in which an optical window that is formed by a transparent body is tilted in a secondary scanning direction and the ghost light due to the reflection at a front surface or a rear surface of the optical window is not allowed to reach a medium that is subjected to scanning.
Moreover, a technology in which the ghost light is removed from an optical axis by tilting a cylindrical lens is disclosed in Japanese Patent Application Laid-open Publication No. H6-82620. A technology in which a scanning lens system is allowed to be decentered so that the ghost light doesn't reach the medium subjected to scanning is disclosed in Japanese Patent Application Laid-open Publication No. H7-230051. Further, a technology in which the scanning lens system is allowed to be decentered so that the ghost light doesn't reach the medium subjected to scanning is disclosed in Japanese Patent Application Laid-open Publication No. 2001-264665.
An improvement in an output speed has been sought. To meet this demand, an improvement has been made in the rotational speed of the rotating polygon mirror in the optical writing unit. However, there are limitations on the improvement in the rotational speed, and the speed has been raised by using a plurality of light sources (combination of laser diodes (LDs)) and a plurality of light emitting points (LD Array). Various structures of light source have been proposed so far. In a case of a multi-beam light source that causes a plurality of light beams to scan the photosensitive drum simultaneously, a method of adjusting the distance between the light beams has been complicated.
In the method of combining a plurality of laser beams by using the beam-combining unit, there are many advantages from the point of view of shortening the wavelength and reducing the cost. Moreover, in cases of making an attempt to adjust and to set the distance between the beam spots (in other words beam pitch: distance between the scanning lines) in the secondary scanning direction in the surface that is subjected to scanning, there is an advantage that the adjustment and the setting can be made by deflecting minutely the direction of emission of each laser beam. However, when the semiconductor laser array is used as a light source, the semiconductor laser array is to be turned roughly around the optical axis to change the distance between the beam spots. Therefore, the amount of light is insufficient due to the distribution of the angle of divergence of the laser beam that is emitted from the semiconductor laser array and it has been difficult to reduce the diameter of the beam spot of the surface to be scanned to a smaller diameter.
When the method of combining the laser beams by the beam-combining unit and the method of using the semiconductor laser array as a light source are compared, in a case of the method of combining the laser beams by using the beam-combining unit, due to the environmental changes and changes in the elapsed time, the direction of emission of each laser beam tends to change easily and the distance between the beam spots in the surface subjected to scanning changes.
In the four-drum tandem image forming apparatus, for example, in a full color copier, four photosensitive drums corresponding to four colors viz. cyan (C), magenta (M), yellow (Y), and black (K) are disposed in a row along a carrier surface of a transfer belt. Further a beam scanner that is provided corresponding to each photosensitive drum, scans beam on each photosensitive drum, forms an electrostatic latent image on a surface of each photosensitive drum, as well as visualizes each of the electrostatic latent images with toner of the corresponding color. The visualized images are transferred by superimposing one after another on a sheet that is carried by the transfer belt, thereby forming a multicolor image. Due to this, if there is a shift in a scanning position in an independent secondary scanning direction for each color, the image quality is deteriorated and there is a color shift.
According to Japanese Patent Application Laid-open Publication No. H10-215351 and H9-189873, it is quite possible that the designing of the adjustment sensitivity is difficult. Further, according to Japanese Patent Application Laid-open Publication No. H10-282531, there has been a possibility of occurrence of problems such as a big size of the apparatus and high voltage for driving. According to Japanese Patent Application Laid-open Publication No. 2002-174785, there has been a need to dispose two dioptric systems in one optical path.
When a structure is made according the technology disclosed in the Japanese Patent Application Laid-open Publication No. H9-274134, if the cylindrical lens is inclined due to assembling error, the light reflected inside the lens becomes ghost light. This problem is described in detail in embodiments of the present invention.
If the method according to the technology disclosed in Japanese Patent Application Laid-open Publication No. H6-82620 is used, the lens optical axis of the light beam is shifted (decentered) due to inclined cylindrical lens, thereby deteriorating the wave front aberration. This results in thickening of the diameter of the beam spot.
The technology disclosed in Japanese Patent Application Laid-open Publication No. H7-230051 is regarding the scanning optical system from the deflector (polygon scanner) onward, and the ghost light due to an optical element before the polygon has not been considered.
The technology disclosed in Japanese Patent Application Laid-open Publication No. 2001-264665 is regarding the reflection between the scanning optical elements from the deflector onward, and the ghost light due to optical elements before the polygon have not been considered.
Moreover, a method of easily adjusting the distance of each light beam in a case of using a plurality of scanning light beams has been sought.